How to Destroy the Universe (11 page)

The fringes form as the light waves from each slit collide with one another at the second screen and overlap, rather like the way ripples on water can overlap. When this happens on water, the two waveforms add together. So where the peaks of two waves coincide, a large peak is formed (called constructive interference); where the dips, or troughs, of two waves meet, a deep trough results (constructive interference again); and where a peak and a trough of equal size
meet, the two simply cancel one another out (known as destructive interference). This principle, in which waveforms add together, is known as superposition.

Exactly the same thing happens with light rays at the screen in Young's experiment. A ray of light is a wave, like a wave on a string, with peaks and troughs. Where a wave peak in the light ray from one slit coincides at the screen with a peak in the light ray from the other, a bright fringe is formed. And the same thing happens where two troughs meet. But where a peak and a trough coincide the two light waves cancel one another out to form a dark band.

Bands are formed because the light from one slit falling on the second screen will generally have traveled a different distance from the light from the other slit. Right in the middle of the second screen both light rays have traveled exactly the same distance, so their peaks and troughs both overlap exactly. There is thus constructive interference, resulting in an overlapping bright image of both slits—i.e. a bright band. Look to either side of the bright central band and you reach a point where the ray from one slit has traveled half a wavelength less than the other. These rays then interfere destructively, resulting in overlapping dark images of each slit—i.e. a dark band.

Of course, this all assumes that the light waves were all in lockstep with one another when they passed through both the slits. In other words, a wave peak passes through one slit at the same time as a wave peak passes through the other, while a trough passes through one at the same time as a trough passes through the other. Physicists call light that has this property “coherent.” If, on the other hand, the light was incoherent, its waves would be hopelessly out of synch and there would be no chance of witnessing any interference fringes. Most light sources in nature are incoherent. When Thomas Young first performed the double-slit experiment in the early 19th century, he was able to generate reasonably coherent light by shining an incoherent source through a pinhole. Because the pinhole is to all intents and purposes a dot of zero size, it filters out the incoherent variations from point to point along the light's wavefront. But this wasn't the only demand placed on the light used in Young's experiment. It was also crucial that the light should be made up of waves of just one wavelength (see
How to make the loudest sound on Earth
).

If light of all different wavelengths is passing through the slits, each wavelength will make a different interference pattern on the screen, with different spacings between the fringes, and the clean pattern of bright and dark bands is lost. Instead, the light must all be of a single wavelength. Because the wavelength of a light beam is what determines its color (for example, red light has a wavelength of 650 billionths of a meter while the wavelength of blue light is shorter, at 450 billionths of a meter), light of a single wavelength is called monochromatic (from the Greek “mono,” meaning single, and “chroma,” meaning color). Making monochromatic light is easier said than done. Ordinary light sources—such as filament lightbulbs—give off light at a large range of wavelengths. The range is determined by the temperature of the source, in this case the hot filament inside the bulb. German
physicist Max Planck used quantum theory (also known as quantum mechanics) to work out the spectrum of radiation from a hot body. The spectrum is just a graph with the range of wavelengths of the electromagnetic radiation given off along the bottom and the intensity of the radiation at each wavelength plotted vertically. For a filament bulb most of this radiation is at infrared and visible light wavelengths, but there's a large spread either side.

Young overcame this problem by using light from a mercury vapor lamp. This is a source of monochromatic light that operates on the principles of quantum theory—although Young didn't realize it at the time, because the theory was yet to be formulated. It works using a glass bulb filled with mercury vapor, through which a large electric current is passed. Energy from the current is absorbed by the mercury atoms, which makes electrons in the outer shell of each atom jump up to a higher energy level. Energy levels are one of the key features of quantum theory, and their existence was one of the predictions of the Schrödinger equation which lies at the theory's heart (see p.116). At very small scales, the energy of an electron in an atom is only allowed to take one of a discrete range of values. An atom raised to a higher energy level soon drops back down again, re-releasing the energy as a packet of light—called a photon. The energy of the photon is determined by its wavelength, so electrons all dropping down from the
same energy level will have the same wavelength—in other words, they are monochromatic.

Nowadays, scientists perform demonstrations of the double-slit experiment using lasers. The word laser is an acronym, standing for “light amplification by the stimulated emission of radiation.” A laser works in a similar way to a mercury lamp. A material known as the lasing medium—ruby is a common choice—is first pumped with energy from a source such as an ordinary flash tube. This raises electrons in the lasing medium to a particular energy level. As they start to drop back down, monochromatic light is released with a characteristic wavelength given by the gap between the energy levels involved. When a photon released by an atom in the lasing medium passes near to another atom with an electron in an energized state, it can trigger the electron to drop down and release another photon. Not only does this new photon have exactly the same energy and wavelength but the peaks and troughs of its waves are naturally synchronized, making it coherent too. This process is called “stimulated emission,” the theory of which was developed by Albert Einstein in 1917.

The laser was invented by US scientist Charles Townes in the late 1950s. It consists of a cylinder of the lasing
medium with mirrors at each end to bounce photons back and forth inside it, so their numbers become amplified by stimulated emission. One of the mirrors is only half-silvered, allowing a proportion of the light to escape as a tightly collimated beam. Lasers proved to be one of the greatest inventions of the 20th century, underpinning devices such as CD players, fiber optics and high-capacity data storage. Today, you can buy cheap lasers as pointers and even key fobs. You can even recreate Young's experiment by plucking a hair from your head and shining a laser at it. The hair acts as the obscuring gap between the two slits, and on the wall behind it you will see a pattern of bright and dark fringes.

When Young produced interference fringes in 1801, it seemed like fairly conclusive evidence that light is a wave. But here's where it starts to get really interesting. In the late 19th century and early 20th century, experimental observations began to roll in suggesting that light can still exhibit some degree of particle-like properties. Max Planck's accurate explanation of the radiation from hot bodies relied on this—as did Einstein's explanation of the photoelectric effect (see
How to harness starlight
). So what was going on? Surely light had to be either one thing or the other?

To get the bottom of the mystery, experimentalists resolved to repeat Young's experiment—but with a twist. This time, rather than shining a whole beam of light through the apparatus, they would send just one particle—that is, one photon—at a time. Common sense would dictate that this single particle of light can only pass through one slit or the other, and so it should be impossible to generate any kind of interference from light passing through both slits. What actually happens is quite remarkable. The scientists fired a photon through the apparatus and recorded the position of the resulting dot on the second screen. Then they repeated the process over and over. As time passed, and more dots accumulated on the screen, a pattern began to take shape—the original interference pattern. But how can this be? At any one time there was only one photon in the system. There's nothing else there for it to interfere with. The only way a single photon can give rise to interference is if the photon somehow interferes with itself. Put simply, it must pass through both slits—it is in two places at once.

This single experiment laid bare the inherent weirdness of the quantum world. Down at this level there is no such thing as a pure particle that is in one place at one time, or a pure wave that is spread out in space—just a strange mixture of the two. Today, physicists
interpret the wave aspect as a wave of probability. Peaks of the wave correspond to where you're most likely to find a particle when you actually make a measurement. A probability wave passing a point in space makes it more likely that you'll find a particle at that point—in much the same way that a crime wave in your neighborhood makes it more likely that a crime will be committed there. In the double-slit experiment, the probability wave passes through both slits and interferes with itself on the screen to create an interference pattern, the peaks of which are where each photon particle is most likely to be found. In 1926, Austrian physicist Erwin Schrödinger drew upon observations such as the double-slit experiment to construct a now-famous equation governing how a particle's probability wave behaves. And from this equation would follow the rest of quantum theory, one of the landmark achievements in 20th-century physics.

• The universe next door

• Many worlds

• Decoherence

• Quantum suicide

• Quantum immortality

• Live long and prosper

From tales of the mythical fountain of youth to drugs that counteract the natural aging process, the quest for immortality has long fascinated scientists and philosophers. It's a field of research that's usually more at home in the labs of medical researchers than the minds of theoretical physicists. But now a physicist has found a way by which it might be possible to live forever—by hopping between parallel universes.

Parallel universes make for great science fiction stories. But in real-world science, they were something of a nebulous concept for many years. During the 1920s and '30s—the years following Albert Einstein's formulation
of the general theory of relativity—physicists discovered wormholes: bridges linking the space and time of our own Universe with those of others. But no one had much clue where these other universes actually were. Are they part of the same space and time as our Universe, or totally disconnected from it? Neither did anyone know whether they obeyed the same laws of physics as our Universe, or different ones. Would they contain galaxies, stars and planets like ours—and maybe even people—or are they barren and empty? Why are these parallel worlds even there in the first place? And that's if they exist at all—no one was even sure whether they were real or just a curiosity thrown up by the complex mathematics of Einstein's theory.

That began to change in 1957 when US physicist Hugh Everett came up with a new way of thinking about quantum theory, the physics governing subatomic particles. Quantum theory supposed that solid particles don't always behave like solid particles, but can also exhibit properties more reminiscent of wave motion. Particles, it seemed, could do things that it had been thought up to that point only waves can do—such as diffracting (spreading out) as they passed through narrow slits, and interfering with one another like ripples overlapping on the surface of a pond. Quantum ripples are interpreted as probability waves. The form of the wave describing a particle is known as its wave-function: an undulating surface covering the whole of
space, with the height of the undulations at any point giving the likelihood of finding the particle there when a measurement is made.

Up until the time of Everett's work, most physicists believed in the so-called Copenhagen interpretation of quantum theory, according to which the act of measuring a particle causes its wavefunction to “collapse”—so that rather than a wave spread out through space the measurement reveals a solid particle with a definite location. In this view, quantum probabilities are simply due to the measurer's ignorance of the state of the quantum particle.